Magnetohydrodynamics (hereinafter abbreviated as MHD) is the branch of physics which deals with the interactions between ionized gases and magnetic fields. Prior MHD generators operate by passing a flow of partially ionized gas, a plasma, through a transversely oriented magnetic field. The interaction of the electrons in the plasma with the magnetic field creates a current flow nearly perpendicular to both the gas flow and the magnetic field. When the gas is contained within a duct or channel whose walls are electrically isolated from each other, a DC electrical potential is formed on the walls. The charged walls (electrodes) can be connected to an external electrical load.
MHD power generating systems typically operate on an open cycle. The extremely high temperatures necessary to form a plasma are generated by burning any of various fossil fuels in heated air or oxygen. The plasma in prior MHD generators of this type is produced by thermally induced dissociation of gas molecules or atoms into free electrons and ions.
The level of ionization in the plasma is governed by the Saha equation: ##EQU1## where: N.sub.i /N.sub.n =ratio of ions to neutral gas molecules or atoms
U.sub.1 =ionization temperature of the gas PA1 K=Boltzmann's constant PA1 T=temperature in degrees Kelvin
Thus in these prior MHD systems the degree of ionization is highly dependent on the gas temperature. This form of ionization is termed "equilibrium ionization" as recombination of the plasma is balanced by the creation of new charged particles. Very high gas temperatures have heretofore been required in order to achieve an adequate degree of equilibrium ionization. This has complicated the structure and added to costs, by dictating the use of expensive refractory materials for example, and has also made it difficult to utilize economical low grade fuels.
Even at the highest working temperatures (approximately 2730.degree. C.), the amount of ionization produced by thermal dissociation of combustion gas molecules is insufficient to raise the conductivity of the gas to a level usable in an MHD generator. Therefore "seed materials" which ionize more easily, such as the alkali metals, are added. In practice less expensive alkali salts are used rather than pure metals.
However the addition of alkali salts such as potassium carbonate, potassium sulfate, or potassium hydroxide causes problems. The amount of seed material that needs to be added is substantial, typically 2-10% of the input fuel weight. Since these seed materials are expensive, they must be recovered. Without seed recovery, prior MHD generators of this kind will not be cost effective compared to conventional fossil fueled steam electric generating plants. Satisfactory seed recovery technology has not yet been developed, at least for large scale operations.
MHD power generators are being proposed as topping cycles to conventional steam electric power plants. In such an application the hot gases which are used in the MHD channel are produced by the direct combustion of natural gas, coal, or fuel oil. Direct combustion of gas or oil in an MHD generator system is straightforward although such fuels are costly. The direct combustion of coal generates fly ash, which at the high temperatures used in MHD, melts and forms a slag coating on the inside walls of the MHD channel. The slag reduces the efficiency of the MHD channel and can cause premature failure of the channel.
As an alternative to direct combustion, solid fuels such as coal or biomass (e.g. wood, agricultural wastes or the like) or municipal solid wastes can be converted to a gas, which can be burned to provide the hot gases for the MHD channel. In the simplest type of gasification process, air gasification, carbonaceous fuels are partially combusted with air to generate a low energy gas rich in carbon monoxide and hydrogen, with an energy content of approximately 6.0 MJ/m.sup.3. Although the low energy gas has only about one sixth the energy content of natural gas (approximately 37 MJ/m.sup.3), the low energy gas can be used to fuel boilers, gas turbines and internal combustion engines. However due to the low energy content of the gas, it must be used near the site of its production. Air gasifiers for both coal and biomass fuels are in commerical production.
A more complex gasification process, oxygen gasification, utilizes pure oxygen to produce either a medium energy gas (12.9 to 13.8 MJ/m.sup.3) or a "pipe-line quality" high energy gas (also called syngas) with an energy content similar to that of natural gas. Although medium or high energy gas can be transported long distances in existing natural gas transmission pipelines, it is more costly to produce than low energy gas. Also, oxygen gasifiers for medium and high energy gas operate at lower energy conversion efficiencies than the simpler air gasifiers for low energy gas.
Thus steam electric power plants of this kind are subject to cost and efficiency problems which can be alleviated if a less costly MHD topping cycle, adaptable to any of a variety of fuels and which does not require seeding with alkali metals, is available.
The present invention is directed to overcoming one or more of the problems set forth above.